Dogs over fertilize nature reserves additionally
Nature reserves also attract dog owners. But the four-legged friends' droppings have consequences for the ecosystems.
Many ecosystems in industrialized countries suffer from overfertilization: nitrogen compounds from agriculture or traffic are introduced via air and water. Species that depend on nutrient-poor conditions are displaced as a result. A research group led by Pieter De Frenne of Ghent University has now identified an additional source of excessive nitrogen input in nature reserves: dogs cause further eutrophication via their excreta, the team writes in "Ecological Solutions and Evidence."
According to the study, dogs contribute eleven kilograms of nitrogen and five kilograms of phosphates per hectare to reserves around Ghent, write De Frenne and co: substantial amounts that further aggravate the nutrient problem in the biotopes and alter the species composition. By comparison, between 5 and 25 kilograms of nitrogen are deposited in nature in Europe via agriculture and transport, depending on the region. "We were surprised at how much dogs can contribute to eutrophication," De Fenne says.
For its study, the team noted the number of dogs in four nature reserves around Ghent during 487 monitoring sessions over 18 months. In this way, the researchers derived how many dogs per hectare per year were present in the areas and how often they defecated there. They then related this to nutrient levels in dog feces and urine recorded in other studies. From this, they in turn modeled different scenarios of fertilizer input.
If, for example, all dogs walked on a leash, the input was reduced in the area and concentrated along the paths: up to 175 kilograms of nitrogen and 73 kilograms of phosphorus compounds would be added per hectare. The input would thus exceed any limit values for the fertilization of agricultural land. If all owners collected the solid waste left behind by their four-legged friends, the input of nitrogen would be reduced by 56 percent and that of phosphorus compounds by 97 percent: they come almost entirely from the excrement. The working group limits that the number of dogs, but not their excrements could be precisely recorded: Their volume depends, among other things, on the size of the animals, which leads to considerable differences depending on the breed.
For nature reserves, these volumes pose additional problems. "In many conservation areas, management is specifically aimed at lowering soil nutrient levels to promote plant and animal biodiversity. This can be accomplished through methods such as mowing and hay removal," says De Frenne. "Our results suggest that currently unaddressed input from dogs in conservation areas could delay these restoration goals."
The scientists urge area managers, especially in nutrient-poor ecosystems, to alert visitors to the negative impacts of dog manure and ask them to remove the feces. In addition, leash requirements should be more strictly enforced. While the negative impact of cats on wildlife has long been known, that of dogs has received less attention. However, free-roaming dogs have been shown to disturb wildlife when they roam the forest or beach far from their owners.
Spectrum of Science
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